In making sub-micron metal-oxide-semiconductor field effect transistors (MOSFETs), it is desirable to provide the transistor with ultra shallow (depths of less than 150 nm) source/drain regions (junctions). The shallow junctions have lower sheet and contact resistance at low leakage currents. Silicided shallow junctions have been found to lower both metal-to-diffusion resistance as well as diffusion sheet resistance and, because of the low dose required, they have the potential to reduce substrate damage associated with ion implantation. Heretofore, shallow silicided junctions have been formed by forming along a surface of a substrate of single crystalline silicon a layer of a metal silicide, such as a silicide of cobalt, titanium tungsten, tantalum or molybdenum. The silicide layer is doped with a desired conductivity impurity by ion implantation. The device is then heated to diffuse the dopant from the silicide into the substrate to form a shallow junction. This process is described in U.S. Pat. No. 4,788,160 (R. H. Havemann et al., issued Nov. 29, 1988) and U.S. Pat. No. 4,816,423 (Havemann, issued Mar. 28, 1989). A low implantation energy of a high dose implant (typically 5.times.10.sup.15 impurities/cm.sup. 2) is used to confine the implantation of the dopant entirely to the silicide. Since the dopant is then diffused into the substrate, there is no implantation damage to the substrate which must otherwise be annealed out.
However, despite the absence of implantation damage in the substrate, the leakage and breakdown characteristics of outdiffused junctions are often not satisfactory. This is especially true when the process is used in conjunction with low thermal processing. For example, insufficient outdiffusion from the silicide in conjunction with a rough silicide/silicon substrate interface can cause silicide spiking. This results in the formation of Schottky diodes which degrade the junction leakage. In addition, at high doping levels and extreme shallow junction depths, the risk of soft breakdown via tunneling also increases.
One known technique which has been attempted to overcome some of these problems is to increase the thermal cycle so as to promote the outdiffusion. However, the silicide must be thermally stable for the anneal cycle required to drive the dopant into the silicon from the silicide. Quite often the silicide agglomerates, thereby causing the interface roughness to increase. This promotes the silicide spiking, which is undesirable. Also, other processing constraints and the device design may limit the thermal cycle which can be used.
Another known technique is the implantation of the dopant tail through the silicide into the silicon. This technique reduces the amount of dopant that must be supplied by outdiffusion from the silicide in order to form a good junction. However, it has been found to be very difficult to control this technique since the implantation tail is very sensitive to variations in silicide thickness. Also, it is very dependent on silicide morphology due to channeling. Another disadvantage of this technique is that the concentration of the implantation peak and implantation tail are not independent of each other. Thus, the amount of dopant needed for outdiffusion from the silicide (i.e., the implantation peak) and the amount of dopant at the silicide/silicon substrate interface (i.e., the implantation tail) cannot be optimized independently.
Still another known technique is to have the implantation peak near the silicide/silicon substrate interface. This technique maximizes the ion-beam mixing effect which results in a spiking. However, considerable metal knock-on into the silicon substrate from the metal silicide as well as crystal damage at the junction are the drawbacks of this technique. Also, the junctions are fairly deep because of the struggle created at the implantation conditions and the extended annealing required to remove damage. This limits this technique to relatively deep junctions (greater than 150 nm).
A further known technique is to deep implant through the silicide. In this technique the full dose of the dopant is implanted at a high energy through the silicide into the silicon. This technique is thereby restricted to deep junctions only and, like any high-dose implant, it generates severe crystal damage in the substrate. Also, extensive annealing is required to remove the implantation damage.